Piezoelectric crystalline film of zinc oxide and method for making same

ABSTRACT

Piezoelectric crystalline film on a substrate, which is a crystalline zinc oxide film with a c-axis substantially perpendicular to the substrate surface, the crystalline zinc oxide film containing, as an additive element, phosphorus. The piezoelectric crystalline films have high resistivity and a smooth surface, and make it possible to produce piezoelectric transducers with good conversion efficiency. Such films are made by sputtering zinc oxide and phosphorus onto a substrate from a film material source consisting essentially of a ceramic of zinc oxide containing phosphorus.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to piezoelectric crystalline films. Moreparticularly, it relates to piezoelectric crystalline films comprisingzinc oxide having a hexagonal crystal structure and a method for makingthe same.

There are many methods for making piezoelectric crystalline zinc oxidefilms such as, for example, vacuum deposition methods, epitaxial growthmethods, sputtering methods, and ion plating methods. Among thesemethods, the sputtering methods, particularly, the radio-frequencysputtering method has been used very often recently because it has theadvantage that a high growth rate of the oriented crystalline films ispossible, thus making it possible to mass-produce piezoelectriccrystalline films, industrially.

When making a piezoelectric crystalline film of zinc oxide on asubstrate surface by the radio-frequency sputtering method, a ceramic ofhighly pure zinc oxide has been conventionally used as a source of thefilm material. However, even when radio-frequency sputtering is effectedwith such a film material source, it results in the formation of acrystalline film which is poor in the adhesion to a substrate and low inthe quality. In addition, it is difficult with such a film materialsource to make a piezoelectric crystalline film with a c-axisperpendicular to the substrate surface. If a piezoelectric crystallinefilm of zinc oxide has poor adhesion, various disadvantages occur. Forexample, when manufacturing an acoustic surface wave filter with such azinc oxide film, it is difficult to form interdigital transducers on thefilm surface, and the produced acoustic surface wave filter tends tohave the disconnection of interdigital transducers, and possesses alarge propagation loss of acoustic surface waves. Also, if the c-axis ofthe zinc oxide film is inclined with respect to the axis perpendicularto the substrate surface, the electromechanical coupling factor islowered, thus making it difficult to produce a piezoelectric crystallinefilm transducer with good conversion efficiency.

It has now been found that the incorporation of phosphorus into a zincoxide film makes it possible to produce a piezoelectric crystalline filmwith a c-axis perpendicular to the substrate surface and with highquality.

It is an object of the present invention to provide an improved zincoxide piezoelectric crystalline film which overcomes the aforesaiddisadvantages.

Another object of the present invention is to provide a method formaking such improved piezoelectric crystalline films.

According to the present invention, there is provided a piezoelectriccrystalline film on a substrate, the film being a crystalline zinc oxidefilm with a hexagonal crystal structure and a c-axis substantiallyperpendicular to the substrate surface, characterized in that said zincoxide film contains phosphorus.

Since the content of phosphorus has great influence on the electricaland physical properties of the films, it is preferred to limit itscontent to the range of from 0.01 to 20.0 atomic percent when convertedinto the percentage of phosphorus atoms. Because, if the content ofphosphorus is less t han 0.01 atomic percent, the film changes for theworse in its quality and adhesion, and if the content of phosphorus ismore than 20.0 atomic percent, the direction of the crystallographicorientation of the film can not be well controlled, resulting in aworsening of the orientation of the piezoelectric crystalline film.

As a material for the substrate on which a piezoelectric crystallinefilm is formed, there may be used those such as, for example, metal,glass, ceramics, single crystal, resins, rubber and the like.

The piezoelectric crystalline film of the present invention has a c-axissubstantially perpendicular to the substrate surface, thus making itpossible to produce piezoelectric transducers with good conversionefficiency. Further, the piezoelectric crystalline films of the presentinvention have high resistivity and good adhesion, and a smooth surface.

The piezoelectric crystalline film of the present invention may be madeby any sputtering method such as the radio-frequency sputtering method,and the co-sputtering method. A preferred method for makingpiezoelectric crystalline films according to the present inventioncomprises sputtering zinc oxide and phosphorus onto a surface of asubstrate to form a crystalline zinc oxide film with a hexagonal crystalstructure and a c-axis substantially perpendicular to the substratesurface, said sputtering being effected by using a film material sourceconsisting essentially of a ceramic of zinc oxide containing 0.01 to20.0 atomic percent of phosphorus.

The present invention will be further apparent from the followingdescription with respect to examples and the accompanying drawings, inwhich:

FIG. 1 is a diagrammatic view of a radio-frequency sputtering apparatusfor making piezoelectric crystalline films.

FIG. 2 is an electron micrograph of a conventional zinc oxide film; and

FIG. 3 is an electron micrograph of a zinc oxide film of the presentinvention.

Referring now to FIG. 1, there is shown a radio-frequency sputteringapparatus with two electrodes which is used for making piezoelectriccrystalline films according to the present invention. The apparatuscomprises a bell jar 1 in which a pair of electrodes, i.e., a planarcathode 2 and a planar anode 3 are positioned in parallel. Fixed on thecathode 2 is a film material source 4 consisting essentially of aceramic of zinc oxide containing phosphorus. A shutter 5 is positionedbetween electrodes 2 and 3. A substrate 6 consisting of, for example,glass or metal is fixed to the bottom of the anode 3 so as to sitopposite to the film material source. The substrate 6 is heated to atemperature in the range of 20° C. to 600° C. during sputtering. A vent7 and a gas inlet 8 are provided in the bell jar 1.

The radio-frequency sputtering is effected in the following manner:After making it airtight, the bell jar 1 is evacuated through the vent 7to a vacuum of more than 1 ×10⁻⁶ Torr, and then supplied with argon oroxygen gas or a mixed gas of argon and oxygen through the gas inlet 8thereby adjusting the pressure in the bell jar 1 to 1×10⁻¹ to 1 ×10⁻⁴Torr. A radio-frequency voltage is applied between the cathode 2 and thebell jar 1 by a radio-frequency electric power source 9. Electric powerof 2 to 8 W/cm² is supplied to the film material source 4.

The film material source consisting essentially of a ceramic of zincoxide containing phosphorus is prepared in the following manner:

Using powder of Zn0, and Zn₃ (PO₄)₂.4H₂ O as raw materials, there areprepared mixtures to produce ceramics of zinc oxide each having acompositional proportion as shown in Table 1. Each of the mixtures ismilled by the wet process, dried and then calcined at 600° to 800° C.for 2 hours. The presintered body obtained is crushed, milled by the wetprocess with an organic binder and then dried. The resultant powder isshaped into discs with a diameter of 100 mm and a thickness of 5 mm at apressure of 1000 kg/cm² and then fired at 1200° C. for 2 hours to obtainfilm material sources.

The thus obtained film material sources were subjected to measurement ofresistivity and percentage of bulk density d_(s) to the theoreticaldensity d_(t) (d_(s) /d_(t) ×100). The results are shown in Table 1.

                                      Table 1                                     __________________________________________________________________________    Additive   Film material source                                                                      Piezoelectric crystalline film of zinc oxide           Specimen                                                                           (atomic %)                                                                          Resistivity                                                                         (d.sub.s /d.sub.t)                                                                  Orientation                                                                         Resistivity                                      No.  P     (Ω . cm)                                                                      × 100 (%)                                                                     .sup.--X                                                                         δ                                                                          (Ω . cm)                                                                      Quality                                                                           Adhesion                               __________________________________________________________________________    1    --    8.6 × 10                                                                      85.0  5.8                                                                              5.5                                                                              3.5 × 10.sup.3                                                                rough                                                                             bad                                    2    0.0005                                                                              1.1 × 10.sup.2                                                                87.0  2.2                                                                              2.7                                                                              7.8 × 10.sup.6                                                                rough                                                                             bad                                    3    0.05  3.2 × 10.sup.3                                                                95.5  1.1                                                                              1.9                                                                              3.9 × 10.sup.7                                                                smooth                                                                            good                                   4    0.1   4.7 × 10.sup.4                                                                97.8  0.5                                                                              2.6                                                                              2.5 × 10.sup.8                                                                smooth                                                                            good                                   5    5.0   5.6 × 10.sup.7                                                                99.1  3.6                                                                              3.8                                                                              1.3 × 10.sup.9                                                                smooth                                                                            good                                   6    15.0  7.2 × 10.sup.8                                                                98.5  4.2                                                                              5.2                                                                              4.7 × 10.sup.8                                                                smooth                                                                            good                                   7    25.0  6.3 × 10.sup.9                                                                98.3  -- -- --    --  --                                     __________________________________________________________________________

Using the respective film material sources, piezoelectric crystallinezinc oxide films containing phosphorus are made on glass substrates withthe aforesaid radio-frequency sputtering apparatus. The radio-frequencysputtering is carried out under the following conditions: A mixed gas of90 vol % of argon and 10 vol % of oxygen is supplied to the bell jar 1through the gas inlet 8, thereby adjusting the pressure in the bell jar1 to 2×10⁻³ Torr. The glass substrate is heated to and maintained at350° C. The film material source 4 is supplied with 6 W/cm² of electricpower with a frequency of 13.56 MHz.

The c-axis orientation of the thus obtained piezoelectric crystallinefilms was measured with a locking curve method by X-ray diffraction(Ref.: Minakata, Chubachi and Kikuchi "Quantitative Representation ofc-axis Orientation of Zinc Oxide Piezoelectric Thin Films" The 20thLecture of Applied Physics Federation (Japan), vol. 2 (1973) page 84;and Makoto Minakata, The Tohoku University Doctor's Thesis (1974)). Themean value (X) and standard deviation (σ) of the angle of the c-axiswith respect to the axis perpendicular to the substrate surface wereobtained from respective specimens. The results of the measurement ofthe adhesion of the film to the substrate are also shown in Table 1. Theexperiment to determine whether or not the film has sufficient adhesiveforce was carried out by the thermal shock test method 107C ofMIL-STD-202D. A film which peeled off from the surface of the substratewas judged as "bad" and, a film in which cracks were produced was judgedas "passable", and a film which showed no change was judged as "good".

As is evident from Table 1, the crystalline films of the presentinvention have a c-axis substantially perpendicular to the substratesurface, from which it will be understood that the piezoelectriccrystalline films of the present invention possess a largeelectromechanical coupling factor, i.e., good conversion efficiency.Further, the piezoelectric crystalline films of the present inventionare smooth, and have good adhesion to the substrate and very highresistivity.

Specimens Nos. 1 and 3 were photographed through a scanning electronmicroscope at a magnification of 1000. FIG. 2 is an electron micrographof the specimen No. 1 and FIG. 3 is that of the specimen No. 3.

As is evident from these figures, the conventional piezoelectriccrystalline films possess a rough surface (cf. FIG. 2), while thepiezoelectric crystalline films of the present invention possess asmooth surface (cf. FIG. 3).

In the above examples, zinc phosphate tetrahydrate is used as a sourceof phosphorus for making ceramics of zinc oxide containing P, but anyother compound of phosphorus may be employed as a source material.

The use of a film material source containing phosphorus has its ownadvantages, as follows.

When mass-producing piezoelectric crystalline films industrially by theradio-frequency sputtering method, it is necessary to increase thecrystal growth rate as much as possible. To do this, the electric powersupplied to the film material source per unit area thereof is increasedwith the result that the film material source is required to have a highbulk density. This requirement is fully satisfied by the sourcescontaining phosphorus. As is evident from Table 1, these film materialsources have a bulk density higher than that of the conventionally usedsources. Thus, the film material sources containing phosphorus make itpossible to mass-produce zinc oxide piezoelectric crystalline films bythe use of high electric powers.

What we claim is:
 1. A piezoelectric crystalline film disposed on asubstrate, said film consisting essentially of a crystalline zinc oxidefilm with a hexagonal crystal structure and a c-axis substantiallyperpendicular to the substrate surface, said crystalline zinc oxide filmcontaining 0.01 to 20.0 atomic percent of phosphorus.
 2. Thepiezoelectric crystalline film of claim 1 wherein said substrate is madeof a material selected from the group consisting of metal, glass,ceramics, single crystals, resins and rubber.
 3. A method for makingpiezoelectric crystalline films comprising simultaneously sputteringzinc oxide and phosphorus from a film material source onto a substrateto form a crystalline zinc oxide film with a hexagonal crystal structureand a c-axis substantially perpendicular to the substrate surfacecontaining 0.01 to 20.0 atomic percent of phosphorus, wherein saidsputtering is effected by radio-frequency sputtering in an atmosphere ofargon, or oxygen, or a mixture of argon and oxygen under a pressure of1×10⁻¹ to 1×10⁻⁴ Torr, and wherein said film material source consistsessentially of a ceramic of zinc oxide containing 0.01 to 20.0 atomicpercent of phosphorus, said substrate being positioned on an anodeplaced in parallel with a cathode on which said film material source ispositioned, said film material source being supplied with an electricpower of 2 to 8 W/cm².
 4. The method for making piezoelectriccrystalline films according to claim 3 wherein the substrate ismaintained at a temperature in the range of 20° C. to 600° C. duringsputtering.